In the art of growing thin films, it is known to expose a substrate to an oblique incident vapor flux in conditions of limited adatom diffusion and thus grow a columnar microstructure on the substrate.
The optical properties of the resulting microstructure are dependent in part on the material used, the porosity of the microstructure and the orientation of the columns of the thin film.
Hamaguchi et al, U.S. Pat. No. 4,874,664, describe lateral shifting or rotation of the position of the substrate in relation to the vapor flux to create uniform film growth and film layers that have columns with different orientations in the different layers. In Hamaguchi et al, the entire substrate is rotated in between periods of exposure of the substrate to vapor flux, or the substrate is laterally shifted during exposure to vapor flux.
The angle of the incident vapor flux in the prior art tends to be in the range from near 0.degree. to 70.degree. where the angle is measured between the vapor arrival line and the substrate normal, which may be referred to as the polar angle. Where the polar angle is zero, the deposited film is a uniform layer, and does not generate the columnar microstructure. For stationary substrates, results have been published for polar angles reaching close to 90.degree.. The angle of growth of the columns is related in a way poorly understood to the angle of incidence of the vapor flux, but is always observed to be smaller, as measured from the substrate normal, than the angle of incidence.
A paper of Azzam, "Chiral thin solid films", Appl. Phys. Lett. 61 (26) Dec. 28, 1992, has proposed rotation of the substrate while it is exposed to the oblique incident vapor flux to generate a helical microstructure having helicoidal bianisotropic properties. The proposed rotation of the substrate is about an axis perpendicular to the surface of the substrate, which is referred to in this patent document as rotation about the azimuth, or variation of the azimuthal angle. No particular polar angle is specified, though a figure shows an angle of less than 60.degree.. The inventors have attempted to grow microfilm structures by rotation of the substrate in the presence of a vapor flux incident at a polar angle of about 60.degree. and 70.degree.. However, the resulting structure does not show well defined structures.
The paper of Assam is a theoretical paper and fails to provide directions on how to carry out the method in practice. In addition, the patent of Hamaguchi et al provides only one particular microstructure, with limited variation of the columnar growth.